Objective To examine the spatial heterogeneity of soil physicochemical property and microbial characteristic at the county scale and their application in soil comprehensive quality assessment, and offer a theoretical foundation for sustainable use of cultivated land.

Method Surface soil samples of farmland from 47 monitoring units in Gaoyao District, Zhaoqing City, Gunagdong Province were collected. The spatial heterogeneity of soil physicochemical properties such as pH, the contents of clay, organic matter, total nitrogen, alkali-hydrolyzed nitrogen and total phosphorus, as well as soil microbial characteristics including soil respiration, mircrobial biomass, and fungi biomass/bacteria biomass were analyzed combining geostatistics and ArcGIS-related techniques. By employing principal component analysis, correlation analysis and general indicator of soil quality (GISQ) method, we elucidated the influence of different factors on the comprehensive quality of farmland.

Result The nugget coefficients of soil pH, as well as clay, organic matter, dissolved organic carbon, total nitrogen, alkali-hydrolyzed nitrogen, total phosphorus contents ranged from 25% to 75%, indicating moderate spatial autocorrelation, and was affected by both structural and random factors. Among the soil microbial characteristic indicators, the nugget coefficient of soil respiration was 29.4%, indicating moderate spatial autocorrelation, but the nugget coefficients of the total soil microbial biomass, as well as fungi, actinomycetes, bacteria biomass were all greater than 75%, indicating weak spatial autocorrelation and poor spatial structure, and influenced by random factors such as human activities. Soil microorganisms were the primary driving factors of soil quality differentiation of farmland in Gaoyao District, especially the total soil mricrobial biomass, as well as bacterial, fungi, actinomycete biomass. Physicochemical properties such as soil organic matter, total and alkali-hydrolyzed nitrogen contents also had considerable impact on farmland soil quality. Additionally, organic matter, total and alkali-hydrolyzed nitrogen contents were significantly positively correlated (P<0.05). The soil quality of farmland in Gaoyao District was generally in a good level, the spatial pattern was presented as northern hilly area > eastern plain area > central plain area > southern hilly area.

Conclusion At the county scale, the spatial structure of soil physicochemical properties is relatively stable, and soil respiration is a suitable indicator for analyzing microbial spatial variability. Soil microbial biomass and structure exhibit significant spatial heterogeneity at the county scale. The combined application of soil physicochemical property and microbial characteristic indicators in soil quality evaluation can more comprehensively reflect the changes in the farmland quality.